Homebrew PCBs

Todd,

The panda is real.  I had an opportunity to play with and feed seven of them
in China.

Bertho

 

From: Todd F. Carney / K7TFC  Sent: Sunday, April 07, 2013 04:54



On Sat, Apr 6, 2013 at 11:53 PM, Boman33 <boman33@...
<mailto:boman33%40vinland.com> > wrote:
>
> As I wrote, I know it works but the question is why.

Mine is not to reason why, mine is but to do or . . . hey, wait a minute!

Actually, I suspect the viscosity and surface tension of the emulsion has
something to do with how the spin-coating method works. It may also be that
it does end up thinner the farther it gets from the center, but that it
doesn't matter given the use and the pretty-course image resolution
required. Back then, I found myself more concerned about the thickness of
the copper than of the emulsion.

The engineers I worked for had always used 2oz copper clad, and that's what
they wanted me to use. I wanted to use 1oz copper because a) I could get
more yield from the etchant, b) it etched faster, and c) because it etched
faster I could preserve fine lines much easier than with the thick 2oz
stuff. If etching takes too long, it begins to undercut the traces and then
the resist flakes off on the edges of the traces for that reason. Thin
traces, such as dogbones or "sneak-throughs," would just get torn up. But
it didn't matter how much data or how many calculations of current vs. temp
rise of traces of thinner clad, they insisted on 2oz. Neither did it matter
that industry standard by that time (1984) was 1oz except for special
applications. Even Coombs the Revered (author of the standard text on pcb
design) could not move them.

I was able to ramrod the abandonment of the spinner/liquid emulsion hassle
and we started using laminated dry film. In that small industrial setting,
spun-and-flung liquid emulsion was a real bother, but for the hobbyist I
think it's not a bad idea, as long as you can put a spinner together for
next-to-nothing. At some point, the cost of the apparatus (along with the
minor hassle) and the emulsion chemistry makes presensitized pcbs a much
better value (and much easier, too!).

Just curious: is that a real panda in your avatar picture?

73,

Todd
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K7TFC / Medford, Oregon, USA / CN82ni / UTC-8
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QRP (CW & SSB) / EmComm / SOTA / Homebrew / Design





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